A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. Lithographic photomasks are typically transparent fused silica blanks covered with a pattern defined with a chrome metal absorbing film. Photomasks are used, for example, at wavelengths of 365 nm, 248 nm, and 193 nm. A set of photomasks, each defining a pattern layer in integrated circuit fabrication, is fed into a photolithography stepper or scanner and individually selected for exposure.
Degradation of photomasks may occur under some circumstances. Furthermore, photomasks are often rejected after fabrication due to inadvertent errors in the size of one or more features.